Only install PHPStan when we need it

This is to avoid error when installing it on PHP < 7.1

.travis.yml

@@ -36,6 +36,9 @@ script:
     - mkdir -p build/logs
     - php vendor/bin/simple-phpunit -v --coverage-clover build/logs/clover.xml
     - if [ "$CS_FIXER" = "run" ]; then php vendor/bin/php-cs-fixer fix --verbose --dry-run ; fi;
+    # only install phpstan when we need it
+    - if [ "$CS_FIXER" = "run" ]; then composer require phpstan/phpstan --no-progress --no-suggest ; fi;
+    - if [ "$CS_FIXER" = "run" ]; then composer require phpstan/phpstan-phpunit --no-progress --no-suggest ; fi;
     - if [ "$CS_FIXER" = "run" ]; then php vendor/bin/phpstan analyse src tests --no-progress --level 1 ; fi;
 
 after_script:

composer.json

@@ -33,9 +33,7 @@
         "php-coveralls/php-coveralls": "^2.1",
         "friendsofphp/php-cs-fixer": "^2.14",
         "monolog/monolog": "^1.24",
-        "symfony/phpunit-bridge": "^4.2.3",
+        "symfony/phpunit-bridge": "^4.2.3"
-        "phpstan/phpstan-phpunit": "^0.11",
-        "phpstan/phpstan": "^0.11"
     },
     "suggest": {
         "ext-tidy": "Used to clean up given HTML and to avoid problems with bad HTML structure."